Apatite (U–Th)/He thermochronology and Re–Os ages in the Altar region, Central Andes (31°30′S), Main Cordillera of San Juan, Argentina: implications of rapid exhumation in the porphyry Cu (Au) metal endowment and regional tectonics

Abstract

Altar is a large porphyry Cu (Au) deposit located in the Main Cordillera of Argentina, 20 km to the north of the giant Los Pelambres-El Pachón porphyry copper cluster, at the southern portion of the Pampean flat-slab segment of the Andes. Although this region hosts telescoped porphyry-epithermal deposits, the precise temporal relationship between porphyry emplacement, mineralization, cooling, and regional orogenic uplift are still poorly understood. New Re-Os molybdenite ages indicate that Altar orebodies are associated with two magmatic hydrothermal centers: Altar East (11.16 ± 0.06 Ma) and Altar Central (10.38 ± 0.05 Ma) formed at temporally distinct periods. New (U-Th)/He ages from the Early Permian and Late Eocene plutons, and the Middle Miocene subvolcanic stocks associated with Cu-Au mineralization of the Altar region reflect a rapid cooling pulse during the Middle Miocene (15.02 to 10.66 Ma) coeval with a major phase of tectonic shortening and regional uplift. The main pulse of rapid cooling and related tectonic uplift in the Altar region was synchronous with the formation of the hydrothermal systems and resulted in an increased focused metal endowment (Au-Cu grades) due to the telescoping of epithermal mineralization over the rapidly uplifted porphyry system. This 11-10 Ma tectonically triggered exhumation event coincides with the collision of the E-trending segment of the Juan Fernández Ridge with the Peru-Chile trench, at this latitude. Collision and ensuing ridge subduction may have driven a localized pulse of rapid cooling and exhumation of the Main Cordillera, that has not been well documented to the north or south of the Altar-Los Pelambres region.

Andean Mountain Building and Foreland Basin Evolution During Thin‐ and Thick‐Skinned Neogene Deformation (32–33°S)

“…Espinacito fault activity and piggyback basin accumulation continued from ~14 to potentially <5 Ma (Figure 9c; Hoke et al, 2015; Jordan et al, 1996; Pinto et al, 2018). Approximately 14−8 Ma AHe cooling ages from the Los Pelambres‐Pachón region indicate synchronous cooling and tectonic exhumation in the hinterland, potentially triggered by middle Miocene thrusting along the Mondaquita fault (Figure 2; Maydagán et al, 2020). Crosscutting volcanic deposits and intrusions pin the cessation of major thin‐skinned shortening in the Principal Cordillera before ~9 Ma, but broad‐wavelength folds in the crosscutting volcanic units require minor late‐stage hinterland deformation (Cristallini & Cangini, 1993).…”

“…These AHe ages define a younger cooling episode than the results from Los Pelambres‐Pachón but coincide with (1) a ~7 Ma AHe age obtained near the northern termination of the Pocuro fault (~45 km northwest of this study's sample locations); (2) thermal models that indicate ~7 Ma rapid cooling in the western Frontal Cordillera at 30°S; and (3) numerous ~8–2 Ma AHe and apatite fission track cooling ages from the Western Principal Cordillera at 33–35°S (Farías et al, 2008; Maksaev et al, 2009; McInnes et al, 2005; Piquer et al, 2017; Rodríguez et al, 2018). Regional late Miocene–Pliocene cooling ages consistently postdate the emplacement of magmatic or hydrothermal systems by at least several million years and point to a major episode of tectonic exhumation and surface uplift that affected the Andean hinterland from ~30°S to 35°S (Farías et al, 2008; Maksaev et al, 2009; Maydagán et al, 2020; Piquer et al, 2017; Rodríguez et al, 2018). At 32°S, seismicity beneath the hinterland and retroarc domains suggests contractional deformation is still active at depth (<10–20 km; Ammirati et al, 2015; Gregori & Christiansen, 2018; Marot et al, 2014).…”

“…Regional provenance and structural indicators suggest that exhumation and topographic uplift of the Andes initiated by the early to middle Miocene, severely limiting sediment transport from coastal sources during Manantiales Basin accumulation (Cristallini & Ramos, 2000; Hoke et al, 2014; Jordan et al, 1996; Maydagán et al, 2020; Perez, 1995; Pérez, 2001; Pinto et al, 2018; Rodríguez et al, 2018). Paleocurrent indicators are further incompatible with sediment derivation from eastern domains (Figure 3; Jordan et al, 1996).…”

“…Interpretations of regional structures that accommodate crustal thickening and transfer shortening from lower to upper crustal levels remain largely schematic and are complicated by limited exposures near the transition from basement‐involved to thin‐skinned deformation (Figures 1b, 2; Allmendinger et al, 1990; Allmendinger & Judge, 2014; Cristallini & Ramos, 2000; Ramos et al, 2004). However, recently published geochronological data for volcanic and sedimentary deposits along the Chile‐Argentina border refine hinterland chronostratigraphy and sediment source characterization, providing new insights into provenance‐based interpretations of exhumational timing (Jara & Charrier, 2014; Mackaman‐Lofland et al, 2019; Maydagán et al, 2020; Mpodozis et al, 2009; Rodríguez et al, 2018). Broadband seismic data now available for Chile and western Argentina have additional potential to improve interpretations of nonemergent structures and better characterize the interactions between crustal thickening and shallow deformation (Alvarado et al, 2007; Ammirati et al, 2013, 2016; Gans et al, 2011).…”

From Pixels to Deposits: Porphyry Mineralization With Multispectral Convolutional Neural Networks

“…Number of study areas References Sudan 1 [39], [40], [41] Namibia 1 [42], [43], [44] Argentina 4 [45], [46], [47] , [48], [49], [50], [51], [52], [53], [54], [55], [56] Iran 4 [57], [58], [59], [60], [61], [62], [63], [64], [65] , [66], [67], [68], [69], [70] Pakistan 3 [71], [72], [73], [74], [75], [76] , [77], USA 5 [71], [78], [79], [80], [81], [82], [83] [84], [85], [86], [87] , [88], [89]…”